We have demonstrated optical frequency combs (OFCs) featuring low and continuously tunable line spacing in the kilohertz range. The OFCs are generated by pulsed gain-switching of optically injected ...commercial semiconductor lasers emitting at around 1.55 μm. After generation, the OFCs are efficiently densified by pseudo-random-binary-sequence phase modulation. The case of an OFC generated by gain-switching at a repetition rate of 1 GHz has been numerically and experimentally analyzed and the conditions for optimal densification have been found. In this case, the comb has been densified by a factor of 2047 and exhibits a 90 GHz flat and broad spectrum consisting of more than 184,000 spectral lines, separated 488 kHz. Although this densification factor is not the largest that can be achieved with the method, it is more than 100 times larger than previously reported factors for gain-switching OFCs. These results open prospects for the use of potentially integrable gain-switching OFCs in applications requiring sub-megahertz line spacing.
•Dense and broad-band OFCs in pulsed gain-switched optically injected semiconductor lasers.•Continuously tunable line spacing OFCs in the kilohertz range by PRBS phase modulation.•OFCs densified by 2047 with > 184,000 spectral lines separated 488 kHz were attained.•These results open prospects for the use of potentially integrable gain switching OFCs in applications requiring sub-megahertz line spacing.
We propose a pulse shaping and shortening technique for pulses generated from gain switched single mode semiconductor lasers, based on a Mach Zehnder interferometer with variable delay. The spectral ...and temporal characteristics of the pulses obtained with the proposed technique are investigated with numerical simulations. Experiments are performed with a Distributed Feedback laser and a Vertical Cavity Surface Emitting Laser, emitting at 1.5 µm, obtaining pulse duration reduction of 25-30%. The main asset of the proposed technique is that it can be applied to different devices and pulses, taking advantage of the flexibility of the gain switching technique.
We present experiments and numerical simulations of optical frequency combs generated by gain-switching a single mode laser diode when subject to optical injection. Our analysis focuses on the combs ...with a frequency separation <inline-formula><tex-math notation="LaTeX">f_R</tex-math></inline-formula> in the GHz range. We present experimental maps in the parameter space spanned by the detuning and the strength of the optical injection that identify the boundaries between regions with different dynamical states. A rich variety of nonlinear behaviors including injection-locked, unlocked and irregular combs are observed and analyzed. The dynamical state corresponding to an injection locked comb, P<inline-formula><tex-math notation="LaTeX">N</tex-math></inline-formula>, is characterized by an optical phase oscillation with a well defined amplitude in such a way that repeats or changes in 2<inline-formula><tex-math notation="LaTeX">\pi</tex-math></inline-formula> with an <inline-formula><tex-math notation="LaTeX">N/f_R</tex-math></inline-formula> period, where <inline-formula><tex-math notation="LaTeX">N</tex-math></inline-formula> is a natural number. Different regions of locked combs with a tongue shape around detuning values given by multiples of <inline-formula><tex-math notation="LaTeX">f_R</tex-math></inline-formula> appear in the maps. P1 and P2 are the most widespread locked states for large values of the modulation amplitude. As this amplitude decreases a much larger variety of P<inline-formula><tex-math notation="LaTeX">N</tex-math></inline-formula> states with large values of <inline-formula><tex-math notation="LaTeX">N</tex-math></inline-formula> appear in small area regions of the map. Our numerical simulations are in very good agreement with experimental results.
Dual-comb spectroscopy has become a topic of growing interest in recent years due to the advantages it offers in terms of frequency resolution, accuracy, acquisition speed, and signal-to-noise ratio, ...with respect to other existing spectroscopic techniques. In addition, its characteristic of mapping the optical frequencies into radio-frequency ranges opens up the possibility of using non-demanding digitizers. In this paper, we show that a low-cost software defined radio platform can be used as a receiver to obtain such signals accurately using a dual-comb spectrometer based on gain-switched semiconductor lasers. We compare its performance with that of a real-time digital oscilloscope, finding similar results for both digitizers. We measure an absorption line of a H 13 C 14 N cell and obtain that for an integration time of 1 s, the deviation obtained between the experimental data and the Voigt profile fitted to these data is around 0.97% using the low-cost digitizer while it is around 0.84% when using the high-end digitizer. The use of both technologies, semiconductor lasers and low-cost software defined radio platforms, can pave the way towards the development of cost-efficient dual-comb spectrometers.
We experimentally and theoretically investigate the photonic generation of microwave signals using a long-wavelength single-transverse-mode vertical-cavity surface-emitting laser (VCSEL) subject to ...two-frequency orthogonal optical injection. We study if a significant reduction of the linewidth is achieved in the double injection locking regime. In this regime, the VCSEL is subject to optical injection by two master lasers in such a way that stable locking is also observed if only light from one of the master lasers is injected. Our model includes the effect of the injected light reflected at the VCSEL's mirror generalizing previous modeling of reflection-mode optical injection-locked VCSELs. Our model also describes the high coherence associated to stable injection locking states and takes into account phase fluctuations in both master lasers. We observe no significant reduction of the linewidth in the double injection locking regime because the linewidth is mainly determined by the phase fluctuations of the two master lasers.
We present a novel implementation of the "phase reconstruction using optical ultra fast differentiation" (PROUD) technique and apply it to characterize the time resolved chirp of a gain switched ...semiconductor laser. The optical temporal differentiator is a fiber based polarization interferometer. The method provides a fast and simple recovery of the instantaneous frequency from two temporal intensity measurements, obtained by changing the spectral response of the interferometer. Pulses with different shapes and durations of hundreds of picoseconds are fully characterized in amplitude and phase. The technique is validated by comparing the measured pulse spectra with the reconstructed spectra obtained from the intensity and the recovered phase.
We propose the use of a polarization based interferometer with variable transfer function for the generation of temporally flat top pulses from gain switched single mode semiconductor lasers. The ...main advantage of the presented technique is its flexibility in terms of input pulse characteristics, as pulse duration, spectral bandwidth and operating wavelength. Theoretical predictions and experimental demonstrations are presented and the proposed technique is applied to two different semiconductor laser sources emitting in the 1550nm region. Flat top pulses are successfully obtained with input seed pulses with duration ranging from 40ps to 100ps.
We have performed a systematic experimental and simulation analysis of the optical frequency combs generated by gain-switching single mode laser diodes, either free-running or subjected to optical ...injection. The simulations have been produced with a dynamical model consisting of three rate equations that include stochastic noise terms. The model parameters have been extracted from experiments. An exhaustive comparison between the simulated and the experimental temporal profiles and optical spectra generated under a wide range of switching and optical injection conditions has been performed. The excellent quantitative agreement found in the entire range of the explored operating conditions supports the validity of the model for the analysis of the comb generation mechanisms. The detailed maps generated provide guidelines for selecting suitable operating conditions for specific comb characteristics.
The relative intensity noise (RIN) in a two‐section distributed Bragg reflector (DBR) tapered laser emitting around 1060 nm is being studied experimentally. By analyzing the RIN spectra, a resonance ...frequency and damping is obtained. Herein, the dependence of the resonance frequency and the maximum RIN at the resonance frequency is studied as functions of the ridge waveguide (RW) and tapered section currents independently. The low frequency RIN is approximately −162 dB Hz−1, and the RIN peak at currents providing high output power is as low as −154 dB Hz−1. The observed nonlinearity of the squared resonance frequency with the RW section current is attributed to carrier saturation. The measured high values of the resonance frequency, exceeding 2.5 GHz for RW currents higher than 100 mA at a tapered current of 3 A, indicate gain lever enhancement.
The relative intensity noise (RIN) of a two‐section distributed Bragg reflector (DBR) tapered laser emitting at 1060 nm is examined experimentally. Crucial parameters such as resonance frequency and damping are derived through spectral analysis. These findings reveal intriguing nonlinearity in the squared resonance frequency and showcase high resonance frequencies exceeding 2.5 GHz, suggesting gain lever enhancement.
We present a dual-comb spectrometer based on gain-switched optical frequency combs which breaks the current spectral resolution limitation of such systems. The external phase modulation of both combs ...using pseudorandom binary sequences results in a flat dual-comb spectrum that features a line spacing of 3.937 MHz, which can be further reduced. The system, whose acquisition speed is 250 ms, is validated by the measurement of the amplitude response of a narrow (<inline-formula><tex-math notation="LaTeX">\sim</tex-math></inline-formula>360 MHz) Fabry-Pérot filter. This approach opens up the possibility of using gain switching dual-comb systems for high-resolution applications with a simple, cost-effective, and integrable perspective.
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